Each of the testes is divided into lobes, or septae, containing coiled seminiferous tubules lined with spermatozoa-producing cells. Between the tubules are hormone-producing cells called interstitial cells, or cells of Leydig. Testosterone is produced by the interstitial cells. Since the testes-containing scrotum hangs below the body, it has a temperature around 89°F (32°C)—ideal for sperm production, which requires a low temperature. When the scrotum is held too close to the body by restrictive clothing, sterility can result.
The seminiferous tubules are the site of sperm maturation from original germ cells (spermatogonia) to mature sperm (spermatoza). This process begins in puberty and is called spermatogenesis. If a small section of a tubule was removed for observation, the wall would appear thick with a hole, or lumen, in the middle. The outer-most layer of this life saver-shaped cut-out is called the basal lamina. Primitive spermatogonia line the basal lamina and move through the inner layers of the tubule towards the lumen as they mature. Sertoli cells surround the maturing sperm and form tight junctions with one another to closely regulate what nutrients enter the developing sperm. Sertoli cells supply the spermatogenic cells with important ions such as potassium. They also form a blood-testes barrier, which prevents some harmful substances from entering the tubule and spermatogenic cells and entering the man's blood. The unique genetic composition of individual sperm cells would cause an immune system attack on the circulating sperm. Sperm genetic diversity is created in the seminiferous tubule during spermatogenesis.
Spermatogenesis processes spermatogonia to spermatozoa in stages. Spermatogonia undergo mitotic divisions to yield primary spermatocytes that have 46 chromosomes identical to other cells in the male's body. Primary spermatocytes then go through two more divisions—this time meiotic—to form secondary spermatocytes and spermatids. Each final spermatid contains 23 randomly-assorted chromosomes that contain all necessary genetic information.
The final phase of spermatogenesis involves structural change. The sperm cell elongates, forming the long flagellum, or tail, which propels it toward an egg. Chromosomes are tightly packed into the sperm head, and an acrosomal tip appears on top of the head that contains enzymes that help the sperm burrow into an egg. In addition, mitochondria are wound around the flagellum's base to fuel the sperm's journey through the female reproductive tract. This shape change completes maturation of spermatids into spermatozoa, or sperm. However, they are still immotile. Sperm enter the lumen of the seminferous tubules and travel in a very concentrated form to the epididymis. The sperm become mobile after about two weeks in the epididymis and are sent to the vas deferens for storage.
The full maturation of a single sperm takes about 70-80 days. Hence, substances a male is exposed to during that period of time may effect the health of his sperm at the end of that time period. Sperm are always available in healthy males after puberty, because spermatogenesis is an ongoing process with cells in all stages of development existing in different layers of the seminiferous tubules. As many as several hundred million sperm can be produced each day. And one man has approximately a quarter mile of coiled seminiferous tubules which produce all these sperm.
Late spermatogenic stages are dependent on testosterone secreted by the interstitial cells of the testes. At puberty, male levels of luteinizing hormone (LH) are elevated due to increased secretion by the anterior pituitary (AP) gland. LH has also been called interstitial-cellstimulating hormone (ICSH) in men, because it stimulates Leydig cells to secrete testosterone. Follicle-stimulating hormone (FSH) is also secreted by the AP and directs early stages of spermatogenesis. Testosterone from the testes is also necessary for secondary sexual characteristics such as facial and body hair growth, voice deepening, and pubertal genital growth.
- Reproductive System - The Spermatic Ducts And Glands
- Reproductive System - The Male Reproductive System
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